stdbuf: fix cross-compilation

Summary:

Partial fix for https://github.com/uutils/coreutils/issues/6591

The current code declare libstdbuf as a build-dependency of stdbuf as a
workaround to enforce that libstdbuf is compiled before stdbuf. This breaks
cross-compilation, because build-dependencies were compiled for the host
architecture, and not for the target architecture.

The reason this workaround is necessary is that bindeps is available only in nightly at the moment:
https://rust-lang.github.io/rfcs/3028-cargo-binary-dependencies.html

This commit replaces the "build-dependency" workaround with another workaround:
calling cargo manually to build libstdbuf in the build.rs of stdbuf, in order to ensure that libstdbuf is built before stdbuf.

Changes:

- Removed cpp/cpp_build dependencies:

The cpp, cpp_build, and related dependencies were removed because they made cross-compilation in a build.rs file very complex, since you need
to pass proper CXX env variables for cross-compilation, whereas cross-compiling rust code using cargo is quite simple.
Provided Rust implementations for getting stdin, stdout, and stderr pointers.
Switched from C++/cpp macro-based initialization to using the Rust ctor crate for library initialization.

- Remove "feat_require_crate_cpp" which is not needed any more, since stdbuf was the only utility using the cpp crate.

Tests:

This commit fixes e.g. this test:
cross test --target aarch64-unknown-linux-gnu --features stdbuf test_stdbuf::test_libstdbuf_preload -- --nocapture

- The "i686" build of stdbuf was also broken (stdbuf 32 bits, but libstdbuf 64 bits) and test_stdbuf::test_libstdbuf_preload of the i686 builds in github CI serves as regression
test for this issue, no need to add a cross-rs test for aarch64.
- The x86_64 musl build of stdbuf was also broken and was passing tests in CI only because it was compiled with the wrong libc (glibc instead of musl)

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

Cargo.toml

@@ -158,7 +158,6 @@ feat_os_macos = [
 feat_os_unix = [
   "feat_Tier1",
   #
-  "feat_require_crate_cpp",
   "feat_require_unix",
   "feat_require_unix_utmpx",
   "feat_require_unix_hostid",
@@ -185,9 +184,7 @@ feat_os_unix_android = [
 #
 # ** NOTE: these `feat_require_...` sets should be minimized as much as possible to encourage cross-platform availability of utilities
 #
-# "feat_require_crate_cpp" == set of utilities requiring the `cpp` crate (which fail to compile on several platforms; as of 2020-04-23)
-feat_require_crate_cpp = ["stdbuf"]
-# "feat_require_unix" == set of utilities requiring support which is only available on unix platforms (as of 2020-04-23)
+# "feat_require_unix" == set of utilities requiring support which is only available on unix platforms
 feat_require_unix = [
   "chgrp",
   "chmod",
@@ -204,6 +201,7 @@ feat_require_unix = [
   "nohup",
   "pathchk",
   "stat",
+  "stdbuf",
   "stty",
   "timeout",
   "tty",
@@ -220,8 +218,6 @@ feat_require_selinux = ["chcon", "runcon"]
 feat_os_unix_fuchsia = [
   "feat_common_core",
   #
-  "feat_require_crate_cpp",
-  #
   "chgrp",
   "chmod",
   "chown",
@@ -287,6 +283,7 @@ clap_complete = "4.4"
 clap_mangen = "0.2"
 compare = "0.1.0"
 crossterm = "0.29.0"
+ctor = "0.4.1"
 ctrlc = { version = "3.4.7", features = ["termination"] }
 dns-lookup = { version = "2.0.4" }
 exacl = "0.12.0"
@@ -502,6 +499,7 @@ yes = { optional = true, version = "0.1.0", package = "uu_yes", path = "src/uu/y
 
 [dev-dependencies]
 chrono = { workspace = true }
+ctor = { workspace = true }
 filetime = { workspace = true }
 glob = { workspace = true }
 libc = { workspace = true }
@@ -524,7 +522,6 @@ uucore = { workspace = true, features = [
 walkdir = { workspace = true }
 hex-literal = "1.0.0"
 rstest = { workspace = true }
-ctor = "0.4.1"
 
 [target.'cfg(unix)'.dev-dependencies]
 nix = { workspace = true, features = ["process", "signal", "user", "term"] }